Geological Evolution of the Southeastern Red Sea Rift Margin, Republic of Yemen

Geological evolution of the southeastern Red Sea Rift margin, Republic of Yemen

IAN DAVISON Department of Geology, Royal Holloway, University of London, Egham, Surrey, United Kingdom TW20 OEX MOHAMED AL-KADASI Department of Geology, Royal Holloway, University of London, Egham, Surrey, United Kingdom TW20 OEX, and Geology Department, Faculty of Science, Sana'a University, P.O. Box 1247, Republic of Yemen SALAH AL-KHIRBASH Geology Department, Faculty of Science, Sana'a University, P.O. Box 1247, Republic of Yemen ABDUL K. AL-SUBBARY Department of Geology, Royal Holloway, University of London, Egham, Surrey, United Kingdom TW20 OEX, and Geology Department, Faculty of Science, Sana'a University, P.O. Box 1247, Republic of Yemen JOEL BAKER \ SUZANNE BLAKEY I DAN BOSENCE I department of Geology, Royal Holloway, University of London, Egham, Surrey, United Kingdom TW20 OEX CHRIS DART J RICHARD HEATON Cairn Energy, Caim House, 61 Dublin Street, Edinburgh, United Kingdom EH3 6NL MARTIKEN McCLAN MENZIEY S GARY NICHOLS 1 Department of Geology, Royal Holloway, University of London, Egham, Surrey, United Kingdom TW20 OEX LEWIS OWEN ANDREW YELLAND

ABSTRACT tension has taken place across a 75-km-wide and is estimated to have 5-6-m.y.-old ocean zone (P = 1.7) in 6-8 m.y. crust along its axis (see, for example, Izzel- The tectonic evolution of the southeast- The relative timing of volcanism followed din, 1987; Sultan and others, 1992; Fig. 1). ern margin of the Red Sea Rift in western by extension and uplift does not fit conven- Many different models of passive (McGuire Yemen has been investigated using a multi- tional models of passive or active rifting. We and Bohannon, 1989), active (White and disciplinary field study of an east-west suggest that the proto-Red Sea Rift was McKenzie, 1989), pull-apart (Makris and transect between A1 Hudaydah and Sana'a. caused by regional plate stresses that ex- Rihm, 1991), and asymmetric rifting (Dixon Slow subsidence of up to 1 km occurred over ploited lithospheric weakening caused by and others, 1989), as well as low-angle de- the area during a 100 m.y. period before rift- the Afar plume. Appreciable doming only tachments (Voggenreiter and others, 1988), ing. There was a major episode of flood vol- occurred after the main episode of volcan- have been suggested for this region. canism between ca. 30 and 20 Ma, and im- ism, which suggests that magmas extruded With the exception of Menzies and others portant extensional faulting began after the before maximum thermal expansion of the (1991,1992) and Huchon and others (1992), eruption of the volcanic rocks and ceased lithosphere took place. most previous work has focused on either before middle to late Miocene sediments the western side of the Red Sea or the east- and volcanic rocks were deposited uncon- INTRODUCTION ern side in Saudi Arabia. Little systematic formably on top of rotated fault blocks on integrated geologic work had been carried the coastal Tihama Plain. Surface uplift has Usually older continental margins cannot out in northern and western Yemen, except produced the Yemen highlands, whose high- provide clear evidence of the early stages of for reconnaissance maps based on remote est peak reaches an elevation of 3660 m. rifting, because they are either uplifted and sensing images (Grolier and Overstreet, This is attributed to plume heating and completely eroded to basement or are bur- 1978; Kruck, 1983; Kruck and others, 1984; eruption of >3000 m of volcanic rocks. Apa- ied beneath large thicknesses of postrift sed- Ministry of Oil and Mineral Resources, tite fission-track ages indicate early to mid- iments. This paper summarizes the results of 1992). Many papers have focused on the dle Miocene exhumational cooling ages, a study of an east-west transect of the south- magmatism of the Yemen highlands (for ex- postdating the major volcanic phase and eastern margin of the Red Sea, which pro- ample, Moseley, 1969; Civetta and others, contemporaneous with rifting. vides a rare opportunity to study extensive, 1978,1980; Chiesa and others, 1983a, 1983b; Volcanism was accompanied by emplace- well-preserved, onshore exposures of the Capaldi and others, 1987; Manetti and oth- ment of subvertical dike swarms, which gen- early stages of rifting. Emphasis is placed on ers, 1991; Mohr, 1991; Chazot and others, erally strike north-northwest to northwest, the nature and relative timing of crustal ex- 1991; and Chazot and Bertrand, 1993), but broadly parallel to the Red Sea coastline. tension, magmatism, sedimentary deposi- there is a need to integrate these studies Major faults indicate northeast-southwest- tion, uplift, and erosion, and we assess the more fully with the tectonic and sedimentary directed extension. Large granitic sheets applicability of current rifting models. history. Theoretical considerations indicate and plutons (up to 25 km wide) intruded the The Red Sea area is a nascent oceanic that large flood volcanic provinces, like the volcanic rocks. Approximately 30 km of ex- basin, which began rifting in late Oligocene Afar-Yemen region, may be explained by

Geological Society of America Bulletin, v. 106, p. 1474-1493, 16 figs., November 1994.

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Pliocene - Recent Cenozoic volcanic Mesozoic sedimentary Archean Basement • rocks rocks ' reworked in Late • Proterozoic Tertiary Pre-rift Late Proterozoic Major faults / Tertiary granite sedimentary Basement structural trends rocks Figure 1. Geologic map of western Yemen showing main rock types and structures. Adapted from Ministry of Oil and Mineral Resources (1992) and our own data.

the action of mantle plumes (White and The spectacular landscape of the transect bia, is the most striking topographic fea- McKenzie, 1989), but this model has still not is divided into several regions on the basis ture of the Red Sea margin, rising abruptly been rigorously tested against geologic field of topography and structure (Fig. 3). (1) from an altitude of -200 m to >1000 m evidence in Yemen. The Tihama Plain, which is a 40-km-wide above sea level. (3) The northwestern Western Yemen is still seismically active, coastal plain trending north-south, rises Yemen highlands, which form a broad pla- and historic volcanic eruptions have been gently from the Red Sea shores to an teau, reach an altitude of 3660 m at Jabal recorded (McDonald, 1972; Pflaker and altitude of ~200 m in the east, where it Nabi Shuyab (Fig. 4), the highest point on others, 1987; Ambraseys and Melville, 1983; is bounded by the Great Escarpment the Arabian Peninsula. The highest areas Fig. 2). It has undergone late Miocene to (Fig. 3). (2) The Great Escarpment, which correlate closely with the present outcrop of Holocene uplift in an arid climate, which has trends north-northwest-south-southeast the Tertiary volcanic rocks on unextended produced up to 3.6 km of vertical surface and can be traced from southern Yemen crust (Fig. 2), which suggests that the high relief (Fig. 3). northward for >1000 km into Saudi Ara- ground is related to the piling up of thick

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Figure 2. Topographic map of Yemen showing that the highest elevations (>2000 m) closely correlate with main flood basalt field. Peaks reach up to 3660 m. Data from topographic maps at 1:250 000 published by the United Kingdom Ministry of Defence in 1986. Hot spring locations from El Shatoury and others (1979) and earthquake epicenters from Ambraseys and Melville (1983).

volcanic flows coupled with enhanced heat drains into the Red Sea. The maximum the central area of the transect have an av- flow. The Sana'a Basin has an ephemeral thickness of Quaternary clastic sediments erage elevation of —1500 m. This area is centripetal drainage pattern that is centered and volcanic ash that accumulated within dominated by rotational fault blocks expos- on Sana'a at an altitude of 2200 m, with an this depression is only —80 m. Several kilo- ing the prerift Mesozoic sedimentary and vol- outlet into a major wadi system in the south- meters to the west of the unextended volcanic rocks. Some of the extended areas are west part of the basin, which eventually canic plateau, deeply dissected mountains of internally drained, the deepest part of these

1476 Geological Society of America Bulletin, November 1994

GREAT SANA'A BASIN TIHAMA PLAIN ESCARPMENT RIFT MOUNTAINS OF N.W. YEMEN

i) Granitic mountains ii) Central rotated fault iii) Eastern high mountain plateau blocks terrain Alluvial fans forming - Bajada - Dendritic - -Trellis Drainage • —Dendritic >•< Centripetal Drainage Superposed Drainage Drainage Jabal Nabi Antecedent Drainage > Height/m Shuyab 3660m

Figure 3. Schematic section from Sana'a to A1 Salif showing main geomorphological regions and drainage types constituting the landscape of northwestern Yemen.

basins corresponding with the greatest down- granite intruded by Precambrian granite and coarse-grained quartz sandstone with thin throw on the faulted blocks. Mesozoic mafic and felsic dikes. The main (decimeter) quartz clast conglomerate. The The absence of sedimentary rocks within basement structures trend approximately Kohlan Sandstone may be interpreted as a most of the major valleys and the thin rego- north-south to northeast-southwest in the transgressive unit consisting of a lower con- lith cover suggests that denudation rates are northern part of the transect area and do not tinental clastic sequence passing through to probably low and that relatively little sedi- appear to influence the orientation of the shallow marine sandstone, and eventually to ment is stored within the transect area, be- north-northwest-south-southeast-trending marine limestones of the Amran Group. cause it is rapidly transported to the Tihama Red Sea margin (Fig. 1). Carpentier and Lamare (quoted in El- Plain. Such styles of sedimentation support Nakhal, 1987) recorded plant fossils of Li- the view that little syn-Red Sea Rift sedi- Akbra Shale assic age from the Kohlan Sandstone of mentation took place within the mountains Wadi La'ah. during the whole extensional history of this This unit is present throughout the margin. The absence of postrift sedimentary transect area except in the Jabal A1 Dhamir Amran Group fill in the highlands allows direct observation area. It unconformably overlies the base- of the rift floor structure. ment, and the equivalent strata in Saudi This group is composed of massive ma- Arabia have been dated as Permian using rine limestone and thin shale interbeds STRATIGRAPHY OF NORTHWESTERN pollen (Kruck and Thiele, 1983). The unit (Fig. 7). Limestone consists of dark skeletal YEMEN reaches up to 130 m thick, but the average micrite with skeletal wackestone and pack- thickness is —40-80 m. The main rocks are stone. The skeletal allochems, together with Strata exposed in Yemen span the Ar- light gray siltstone and laminated dark gray muddy textures, indicate a shelf environ- chean to the Cenozoic (Fig. 5). These rocks shale, and we interpret them to be fluvio- ment of moderate depth during deposition are described below with reference to lith- glacial in origin (Fig. 6A). of the Amran Group. Ismail (1993) and El- ologic logs compiled at representative Anbaawy (1984) determined a Bajocian to points along the transect. The whole strati- Kohlan Sandstone Kimmeridgian age for the Amran in north- graphic succession is described in detail, be- ern Yemen, whereas Al-Thour (1992) re- cause it provides an important record of the This unit was studied in the transect area ported late Callovian to Tithonian forami- tectonic setting and events leading up to in Wadi La'ah north of A1 Mahwit (Fig. 4), nifera. Major vertical or areal variations Red Sea rifting. where 150 m of medium-grained sandstone were not recognized in the Amran Group in and siltstone, interbedded with dark purple the 100-km-long east-west transect. Precambrian Basement and gray shale, lies directly on granitic rocks The Amran Group records a consistent of the basement (Fig. 6B). Toward the top of water depth during deposition of the whole Basement rocks are generally believed to the section, the sandstones show evidence of sequence, implying the subsidence rate be Late Proterozoic in age, although older bipolar cross-bedding, probably indicating a more or less equaled the sediment accumu- Late Archean ages have recently been ob- tidal influence close to the main contact with lation rate (8 m/m.y.) for some 50 m.y. The tained from gneisses south of the transect, the limestone of the overlying Amran transect area became emergent following a east of Rada (Fig. 1, Whitehouse and others, Group (Fig. 6B). relative sea-level fall, leading to vadose dis- 1994). Rock types include metasedimentary Farther west in Wadi Siham, the lowest solution and cementation before the depo- schist, amphibolite, migmatite, gneiss, and units of the Kohlan are cross-bedded, very sition of the Tawilah Group.

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o ft O, o" 09

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Figure 4. Simplified geologic map of the transect area, indicating major structures and rock types. The two areas mapped in Fig- ures 12 and 13 are shown, and the locality of the seismic line used in Figure 11 is marked. Ak, location of Akbra log in Figure 6A; Kh, location of Kohlan log in Figure 6B; Am, position of Amran log in Figure 7; Ta, position of Tawilah log in Figure 8. Based partly on Kruck and others (1984).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/11/1474/3382029/i0016-7606-106-11-1474.pdf by guest on 28 September 2021 LITHOLOGICAL AGE /Ma PERIOD LITHO- LITHOL OGY THICK ENVIRONMENT TECTONIC EVENTS STRAT DESCRIPTION (m) o- VvV®5; Carbonate/clastic rocks Volcanic j k A "pliocene ~ 1000 Baid Alkali basalt, Plateau Magmatism /iL L L salt, organic 10- Formation Subsiding rich shale and Miocene ."•LjL^ 4000 Sabkha Extension t silst.one. 1 Jv 20- v v Yemen Alkali basalt,rhyolite, Volcanic £< Volcanic ' 1 Oligocene v ignimbrite, tuff <2000 Plateau 30- Group and A-type granite Shallow lakes Magmatism fc Oiachronous 40- conformable contact. Ferricrete Eocene concentrated at top 50- i k

— — — 60- Paleocene Buff-colored, medium 70- to very coarse- Slowly grained, trough cross- subsiding bedded sandstone fluvial plain 80- (quartz arenite). Upper Tawilah 200- & shelf below Conglomerate and sea level Group ;.*.;.'•; very few fine sandstone 400 90- to and siltstone with nodular iron. 100- UJ o fi UJ 110- OC Ü 120- Lower

130-

140- Disconformity with <{•: .<> :•: .i?V. karst. Sandstone Slow sag 150- and Limestone basin 1 1 , Lst. fossiliferous Tithonian - Slowly A mran 1 1 1 Dolomite and shale 300- subsiding 160- Group inter-beds dark gray Callovìan. 1 1 500 shelf below skeletal micrita, sea level O 1 1 1 skeletalwackestone 170- 55 ^^^ and packstone. C

200- wwv 210-

220- TRIASSIC 230-

240" Shale, dark-gray green Glaciogenic 250- Akbra <130 Shale Siltstone, diamictite Shallow water f shale. 260- PERMIAN 270- X 280-

BREAK W TIM ESCALE Archean gneiss terrains Multiple LATE Orogenies >570 Ma PROTERO- Granitoid,Migmatite, and late ZOIC Schist, Amphibolite Proterozoic ARCHAEAN $ island arcs and ophiolites

Figure 5. Lithostratigraphy and tectonic events along northwestern Yemen transect.

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Lithofacies DEPOSITIONAL Codes ENVIRONMENT LITHOLOGIC DESCRIPTION (Eyles et al Height 1983) (m) CLAYS.VFS. —HH—

DIAMICTITE, blocky, green sSi.ltstorie matrix with granitic boulders up to 50 cm diameter Glacial (possibly Dmm with edge rounded, striated and polished surfaces sub-glacial Sm Meter-sized lenses of coarse sand are present or englacial meltout Dmm till)

SHALE - dark gray, planar cm scale bedding with carbonate Fid rich SILTSTONE with cm sized ripple cross laminations

40- Prominent slumped shale horizon.

SHALE, fine-grained parallel-planar laminated rhythmite, dark gray Fid DROPSTONE of small edge-rounded pebbles of granite and coarse sandstone

Starved ripples of fine sandstone Proximal Glaciomarine/ 20- glacio-lacustrine SHALE with coarse SANDSTONE layers containing mm-cm size rip-up clasts Sub-glacial Fl+s GLACIAL STRIATED SURFACE Deformed till SILTSTONE - light-gray green blocky matrix with edge-rounded with syn- Dmms 10- bullet-shaped granitic boulders up to 1 m diameter sedimentary shears Fl SHALE - dark gray Fl SILTSTONE - light gray-green with basement pebbles Subaqueous SHALE - dark gray-green shale, blocky glaciomarine or s.'J CONGLOMERATE with basement pebbles glaciolacustrine Precambrian basement

Figure 6. (A) Lithologie log of Akbra Shale at lat. 15°44'26"N, long. 43°39'41"E. Location marked on Figure 4. (B) Lithologie log of the Kohlan Sandstone from Wadi La'ah 5 km north of A1 Mahwit. Location marked on Figure 4.

Tawilah Group and siltstone make up <20% of the se- marine sediments recorded in southern quence, and shale is uncommon (Fig. 8). A Yemen (Al-Subbary, 1990; Beydoun, 1964). The overlying Tawilah Group is a silici- prominent purple ferruginous sandstone ho- The bulk of the Tawilah Group is inter- clastic-dominated sequence that crops out rizon up to 10 m thick is present in the Shi- preted as a sequence of braided, fluvial- over a large area of northern and central bam area (Shibam Member; El-Anbaawy, channel deposits interbedded in its upper Yemen. Age determinations range from 1985) and can be laterally correlated >100 sections with ferruginous paleosol (ferri- Late Cretaceous to Eocene (Al-Subbary, km along the study transect. The upper crete) horizons (Al-Subbary, 1990; Al- 1990; Al-Subbary and others, 1994). The Medj-Zir Formation contains shallow ma- Subbary and Nichols, 1991; Menzies and group is —400 m thick in the eastern part of rine sandstone with agglutinated marine others, 1992). the transect between Tawilah and Sana'a foraminifera in the lower part, with some Major angular unconformities were not (Fig. 4) but thins westward of Tawilah to nodular beds (Fig. 8; Al-Subbary, 1990). observed within the Tawilah Group despite —150-200 m in the Jabal A1 Dhamir area. Distinct multicolored horizons indicate that its estimated 100 m.y. duration (latest Ju- The group has been divided into two forma- pedogenic processes were responsible for rassic to Eocene). Subsidence was very slow, tions (Fig. 8). The lower Ghiras Formation important hematite concentrations. Conti- but probably sporadic, as suggested by the consists of medium- to very coarse-grained, nental fluvial sandstone and paleosols dom- long periods of nondeposition represented trough cross-bedded sandstone that occurs inate in the uppermost part of the Medj-Zir by the very mature paleosol and ferricrete in decimeter- to meter-thick beds, amalga- Formation, with the latter clearly represent- deposits. mated into units tens of meters thick ing periods of nondeposition and soil devel- The Lahima Member is a distinct unit at (Fig. 8). They are generally mature quartz opment in a semiarid environment. the top of the Tawilah Group in Wadi La- arenites. Conglomerate beds of well- Paleocurrent data, collected from fluvial hima, south of A1 Mahwit (Fig. 9). It consists rounded vein quartz and quartzite clasts are channel deposits in the transect area and of calcareous siltstone with concretions, in- common, mainly at the base of sandstone south of it, suggest flow to the northeast and tercalated with micrite containing gastro- units that reach up to several meters in east-northeast (Fig. 8), consistent with the pods and bivalves; it is interpreted as depos- thickness. Thinner beds of fine sandstone observed eastward facies change to more its of a low-relief continental environment

1480 Geological Society of America Bulletin, November 1994

lenses are within the volcanic pile and are DEPOSITIONAL HEIGHT LITHOLOGIC DESCRIPTION composed of calcareous sandstone, mud- ENVIRONMENT CLAY SILT SAND stone, and reworked mafic and silicic vol- j—^ volcano Jabal A1 Isi the base of the volcanic pile is conformable. acid volcanic rocks in the Sana'a region, and is reported to be dormant (Plafker and oth- finally to 2000-3000 m of dominantly basal- ers, 1987). Yemen Volcanic Rocks tic volcanic rocks (Fig. 10). Volcanic prod- ucts include subalkaline and alkaline basalt, Granitic Rocks Two main groups of volcanic rocks are in basanite, hawaiite, trachyandesite, and the transect area (Civetta and others, 1978; comenditic rhyolitic ignimbrite flows. Silica In the western part of the transect, alka- Capaldi and others, 1983, 1987; Fig. 2): (1) contents of the flood volcanic rocks are line granitic plutons up to 25 km across Widespread Oligocene-Miocene flood vol- markedly bimodal, with samples containing (east-west dimension) crop out sporadically canic rocks yielding K/Ar ages of 18-29 50 ± 4 and 72 ± 4 wt% Si02 (Baker and along the north-south-trending Great Es- Ma are designated here as the Yemen Vol- others, 1993b). Elemental and Pb, Nd, and carpment (Figs. 1 and 4). The plutons may canic Group. These are intruded by gran- Sr isotopie data confirm that most of the be the exposed roots of a chain of caldera ite along the Great Escarpment. (2) The extensive silicic magmas are the product of centers. K/Ar ages suggest that they were area also includes discrete fields of Plio- crystal fractionation from basaltic melts emplaced between 20 and 26 Ma (Civetta cene-Holocene alkaline intraplate volca- (Baker and others, 1993a, 1993b). and others, 1978; Overstreet and others, noes (northwest of Sana'a, Marib, Ataq, The Oligocene-Miocene flood volcanic 1979; Capaldi and others, 1987), which is Dhamar-Rada) (Chiesa and others, 1983b; rocks appear to have been constructed by consistent with similar K/Ar ages of large Capaldi and others, 1987; Fig. 4). eruption of extensive fissure-fed basaltic amounts of silicic volcanic and pyroclastic The Oligocene-Miocene flood volcanic flows, a number of large basaltic shield vol- rocks erupted in the upper part of the vol- rocks increase in thickness westward from canoes, and ignimbrite and ash-fall deposits canic pile. Our field mapping has shown that 700 m of mainly ash-fall tuffs several kilo- from individual calderas. Sedimentary the Great Escarpment is not a continuous

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Figure 8. Lithologie log Tawilah Group X & AGE li o ® LITHOLOGY LITHOLOGIC BIOZONATION INTERPRETATION northwestern Yemen. At Jabal Marmar, lat. M WPG DESCRIPTION (FORAMS) 2S H I I I I 15°35'N, long. 44°24'E, northwest of Sana'a Q- (see Fig. 4 for location). C5 Tawilah Group X sandstone with • Co eroded Amran 3 o; LU clasts at base oo 500-: Emergence Â _Paleokarstic surface Sandy limestone. Varicolored, massive, Shallow marine fine to medium Permocalculus oc TT7 Innoplntus silic ¡clastic/ 3 grained fossiliferous carbonate ferruginous limestone contact metamorphism of the Amran lime- X regressive o < x Whitish-gray fine sequence stone have been observed. The intrusion at t to medium grained Jabal Bura is a porphyritic alkali feldspar K < gs Everti- 400- hard massive,

Gray to yellowish fine to medium Syn- to Postrift Deposits on the Tihama grained friable to Plain and in the Red Sea -58 200 hard laminated Q-UJ X fossiliferous limestone. < Intercalated shale Synrift deposits have not been found o cropping out onshore. Neither wells nor Se cc seismic data give any clues as to the nature of the synrift strata in northern Yemen. Alveosepta powersi However, synrift continental sandstone and fine-grained lacustrine deposits have been 100- recognized farther north onshore in the Jid- Thick massive dah to Jizan area of Saudi Arabia (Brown fossiliferous, marine limestone and others, 1989). Several wells have pene- trated to depths of 3 km on the Tihama TXT Plain and offshore in the Red Sea, and the Everti- Ix oldest encountered sediments associated so cylimmina S sp. with basin opening are a sequence of interbedded, undeformed carbonate and clastic base not seen Key rocks found in the Zaydiyah-1 well (Fig. 4), which have been dated as early to middle ^ stromatoporoids •r\ brachiopods M Micrite P Packstone Miocene (Hughes and Beydoun, 1992). ^ gastropods W Wackestone G Grainstone \j burrows These strata clearly lie beneath a major dis- bivalves • shaley limestone conformity formed at the base of the middle Figure 7. Lithologie log of Amran Formation, from Al-Aydein, Wadi Thula, lat. 15°35'N, to upper Miocene main halite unit imaged long. 43°50'E. Based on A1 Thour (1992) and our own observations. See Figure 4 for on reflection seismic data, which dips gently location. (<5°) westward toward the Red Sea. The relationship of these earliest-drilled strata belt of granite plutons (Blakey, 1994) as in- area of —620 km2 and consists of very to the underlying units is not totally clear as dicated on previous photogeological maps weathered, coarse-grained granite and sy- the tilted fault-block topography, seen on (Kruck and others, 1984; Ministry of Oil and enite. The granite generally has steeply dip- the poor seismic data and at outcrop on the Mineral Resources, 1992), but is also com- ping lateral intrusive contacts and flat-lying eastern edge of the Tihama Plain, abruptly posed of a significant amount of silicic por- upper intrusive contacts with flood basalt falls below the limit of seismic resolution phyritic lava. and the Amran Group. There was little de- westward (Fig. 11). We classify the sediments above the disconformity as postrift The granite in the transect area (Jabal formation of the country rocks during intru- deposits because they do not appear to be Hufash and Jabal Bura) covers a surface sion, and some skarn mineralization and

1482 Geological Society of America Bulletin, November 1994

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Claystone

Siltstone

Sandy mudstone

Iron concretion Cross-bedded fine sandstone o. 3 Medium gr. sst. O :ct:C>: Channel conglom. (D o Coarse gr. sst. c 'CM Conglomeratic sst. CO o uuu Burrows o Benthic Foraminifera >

BASALTIC LAVAS

¡Jijg.-WW-«-.«! ! /

3 o. n=5 CS EC ? fc ít ^ à^ À

r-i- c s f m c p ¡ AMRAN GP Grain size

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Lithology Lithology Lithology

Sand Sand Sand CS f m c CS f m c CS f m c 100 150 Q. y.yyyv LEGEND D y-y-y-y-v yiyiyV* O y:y:y:y\ cc C3 95 145 Basaltic flows yyyy-: :' zO v-Vv v ' y.y. Volcaniclastic rocks < : : v v and tuffs 140 o 90- // Sills and dikes o \ Iron oxide concretion I siltstone Z 135 35- with W 85 ^. Cross-bedding iron oxide ¡= and ¡¿j altered Û. thin shale basalt D so F»:- O 130 Œ (3 O 25 75 < 125 CC siltstone o I u u u'u'i UJ and PK Ferricrete m limestone o hü; 20 2 70 with > 120 E—ïHij Claystone UJ gastropod •y.y.y.y. Iüüh: and and 2 UJ ^y-'y-y-y-* BËHËB siltstone < bivalves UJ !-yyyy- • KJ LJ U U I Claystone 15 > 115 i'Hj'Y »I Y 65 • yy-yy-' fp i-y-y-y-yy-y'y-y-^' Chert >y:y:y'y:' 10-s 60 • 110 Limestone ¡:y:y:y:y:

Sandstone m 55

J ~ 20m yellowish-whitet sandstone

Figure 9. Lithologic log of contact between top of Tawilah Group and Yemen Volcanic Group in Wadi Lahima. Location marked on Figure 13A.

affected by major extensional faulting, other from a few hundred meters of mainly anhy- others, 1994, Fig. 4). Offshore, seismic data than that due to gravity sliding on salt de- drite in the Zaydiyah-1 well to a largely ha- confirm the presence of a wide range of tachments, where extensional strain is bal- litic unit, estimated to have an original thick- halokinetic structures, including large de- anced by contractional strain at the front of ness of up to 2 km in the basin depocenter tached salt canopies now lying 2-3 km above the slides (Heaton and others, 1993). This situated 20-30 km offshore. Seismic data in- their original level (Heaton and others, interpretation differs from that of Hughes dicate that the base of the salt subsided 4-5 1993). The majority of the offshore islands, and Beydoun (1992), who interpreted the km below sea level (Fig. 11). The original such as the Farsan, are formed from reefs entire halite unit as synrift. salt has become diapiric, and five large salt built on the tops of diapirs. Above the initial postrift deposits, a ba- domes reach the surface onshore (El- Above the main evaporite group, a thick sinwide evaporitic unit thickens westward Anbaawy and others, 1992; Davison and cyclic succession of mixed evaporite and

1484 Geological Society of America Bulletin, November 1994

West (rift margin) East

>1000 m proximal mafic and silicic pyroclastic 3600 m asl major ignimbrite flow units 1500 mi rocks i—i welded and unwelded airfall 1— pyroclastic rocks (lithic tuffs)

8— basalt flows 1000 ml

500 ml 0.5-20 m thick subalkaline basalt flows unconformity?

w»^ Tawilah Group: 15°23'30"N 15'24'52"N 15;29|2o;;n cross-bedded fluviatile 43'27'22"E 44°09'52"E ^ 25'08"E sandstone and siltstone 1050 m asl 2300 m asl 2370 m asl

Wadi Lahima section Sana'a section NE Sana'a section (thickness = >1500 m) (thickness = 1500 m) (thickness = 650 m)

Figure 10. Yemen Volcanic Group lithostratigraphy across transect area (Baker and others, 1993a).

clastic rocks of middle to late Miocene age started, because high heat flow is still re- the Yemen volcanic rocks in the east to increases in evaporite content from west to corded in the oil exploration wells. Geother- west-dipping tilted extensional fault blocks east. Paleoenvironmental interpretation of mal gradients reach 50 cC/km onshore in the farther west (Figs. 4, 12, and 13). The ex- this succession suggests a relatively shallow Zaydiyah-1, and up to 80 °C/km offshore tended region varies in width between 80 basin, with periodic connection to the ocean near the continental-oceanic crust transition and 120 km from the Red Sea coastline and filled by clastic input shed by alluvial in the Al-Meethag-2 well (Fig. 4; Barnard (Fig. 4). Within the transect, we have stud- fans and rivers from the east. During this and others, 1992). ied the structural geology of two areas (Bajil time the coastline must have repeatedly un- and A1 Mahwit) in detail (Figs. 12 and 13), dergone cycles of lateral migration of up to STRUCTURAL EVOLUTION OF THE and our structural observations described 100 km, from a position close to the Great EASTERN RED SEA MARGIN, YEMEN below mainly refer to these two areas. Escarpment at the eastern edge of the The tilted fault blocks are typically be- Tihama to a remnant hypersaline lake in the Pre-Red Sea Tectonics tween 2 and 6 km across and generally in- center of the Red Sea. corporate all of the Phanerozoic sedimen- At the end of the Miocene epoch, the Af- Mesozoic rifting in Yemen commenced in tary section and the Precambrian crystalline rican and Arabian plates finally split apart, the Early to Middle Jurassic with the for- basement. Clearly defined stratigraphic and active volcanism began in the center of mation of the Shabwah, Balhaf, and Marib thicknesses allow fault displacements to be the Red Sea Basin. This was marked by a grabens (Fig. 1). The widespread deposition accurately estimated, and displacements are change in sedimentation in the offshore of the Kohlan, Amran, and Tawilah sedi- usually 0.5-2 km with a maximum of 4 km of area, which in the final part of the postrift ments between these grabens indicates that slip estimated on a fault that bounds the phase is characterized by development of a a slow regional subsidence of up to 1 km Tihama escarpment near Jabal A1 Dhamir widespread open marine carbonate plat- occurred from Permian to Eocene time (210 (Fig. 12B). form that reaches up to 400 m thick, with m.y. period). The subsidence occurred over Faulting has rotated beds on average 35°, biostratigraphic affinities with the Indian an area that is too widespread, with an in- with a maximum of 55° in Wadi Lahima, 10 Ocean (Jones and Racy, 1994). In the compatible timing, and too prolonged to be km southwest of A1 Mahwit (Fig. 13). There Tihama area, the sedimentation continued explained by thermal subsidence associated are no significant differences in the rotation to be dominated by alluvial-fluvial process- with the Al-Jawf-Marib graben alone, and of bedding across each major fault plane, es, and a similar pattern continues at the the reason for this slow sag is still not clear. implying that most faults dip consistently in present day. a planar domino style and that faulting oc- The Zaydiyah-1 well (Fig. 4) encountered Red Sea Rift Tectonics: Style, curred after deposition of the exposed se- —3000 m of middle Miocene to Holocene Distribution, and Amount of Extension quence. Striae on the fault planes indicate rocks, indicating that sedimentary loading is mainly down-dip extensional faults. An orig- important (Hughes and Beydoun, 1992). The structure of the study transect passes inal fault dip angle of 60°-65° is indicated by Thermal subsidence has presumably not yet from the elevated and unfaulted plateau of the average exposed bedding/fault plane

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Zaydiyah-1 well w projected along strike from 35 km N of line

TWT sec

Figure 11. Seismic reflection profile across Tihama Plain, showing gently dipping postrift unconformity overlain by middle to late Miocene salt. More steeply dipping reflectors below the unconformity are probably Tertiary volcanic rocks. Location is shown on Figure 4.

cut-off angle, which gives an extension fac- basement, with fewer larger faults observed Huchon and others (1992) used crosscut- tor, p, of 1.6-1.8, calculated with the aver- at basement level. ting relationships of dikes and small-scale age measured bedding dip of 35°. The extended upper crust is elevated up faults to suggest that possibly four distinct Most of the major fault planes are not to 2000 m above sea level indicating that phases of extension occurred, but crosscut- well exposed. Smaller faults with meters to highly extended ((3 = 1.7) rift shoulders may ting relations of major faults are difficult to tens of meters of throw have bedding cut-off be uplifted for several tens of million years ascertain. It is acknowledged that there may angles of —90°, which suggests they may after rifting has ceased. Thermal contraction have been several localized extension direc- have formed after initial rotation of the bed- and erosion may bring the margin back tions, and this is to be expected during most ding. These faults are commonly marked by down to near sea level, and it is possible that rifting events. However, the main crustal sheared dikes, probably indicating that the all traces of extension will have been re- extension in the two areas studied was pro- dikes served as weaknesses subsequently moved, so that the margin would appear to duced by major northwest-southeast-strik- used by faults. Conversely, some dikes in- be narrower and have undergone much less ing faults, indicating northeast-southwest- truded preexisting faults, indicating that at extension. directed extension. least some of the faulting and dike injection The faults dip consistently eastward away occurred simultaneously. Continuous expo- Fault Patterns from the rift axis in the eastern part of the sure of fault blocks up to 5 km across indi- mountain transect (Fig. 12). The faults cates that most of the deformation is taken The location and strike of the faults are abruptly change to a west dip on the west up on the major faults (>100 m displace- well defined by 1:50 000-scale mapping side of the prominent granitic intrusions of ment) rather than being distributed evenly (Figs. 12 and 13). Most of the extensional Jabal Hufash and Bura, however, and along with large numbers of smaller faults. Many faults have a northwest-southeast strike, but the Great Escarpment, which bounds the authors have suggested that small faults are some north-south-striking and rare east- Tihama Plain (Figs. 4 and 12). The direction responsible for a large proportion of the ex- west-striking faults are also present of fault dip changes twice along strike on the tension in sedimentary basins, from obser- (Fig. 13) (Huchon and others, 1992). A Tihama Plain, with each accommodation vations of faults in synrift sedimentary fill northeast-southwest extension direction is zone coinciding with a granitic intrusion. (for example, Sclater and Shorey, 1989). also interpreted in Ethiopia in Late Creta- The nature of the escarpment in the Our observations are restricted to basement ceous to early Tertiary time (Almond, transect area is variable, consisting of a se- and lithified prerift sediments, suggesting 1986a, 1986b), suggesting a widespread ex- ries of extensional fault terraces (for exam- that fault populations may differ between tension occurred throughout the southern ple, Jabal A1 Dhamir) (Fig. 12) that gradu- weakly lithified sediments and crystalline Red Sea area. ally step down toward the Tihama Plain, or

1486 Geological Society of America Bulletin, November 1994

upstanding granite intrusions and porphy- through to the Yemen Volcanic Group. Al- northeast-east-oriented extension occurred ritic acidic lavas that are more resistant than though the surface flows are voluminous, during dike injection, which broadly coin- the surrounding rocks and sediments (for the feeder dikes generally account for <1% cides with the direction of motion of the example, Jabal Bura, Figs. 4 and 12). Field of regional extension, although this may Arabian plate as defined by continental re- mapping indicates that individual fault reach as much as 25% (Mohr, 1991). Dikes constructions and transform fault orienta- scarps bordering the Tihama Plain in the were intruded in equal proportions into the tions (Sultan and others, 1992). Farther Bajil area have been eroded back from their western faulted part of the margin and the north in Saudi Arabia, most dikes (22-25- original position by no more than 1-2 km. generally unfaulted strata and basement of m.y.-old K/Ar ages) are oriented parallel to The main contact between the Precam- the north and eastern part of the transect. the Red Sea coast, following the same pat- brian basement and the Mesozoic strata This suggests that the Cenozoic volcanic tern as in northern Yemen (Bohannon and trends approximately east-west, 5-10 km cover has probably been eroded from the Ettreim, 1991; Coleman and others, 1977). north of Al-Mahwit. This contact forms an basement rather than never having being Near Taym and Musaymir, the dikes are impressive scarp along which a pre-Meso- deposited. The dikes are most numerous predominantly oriented northwest-south- zoic unconformity crops out that dips 10°S. along the 20-30-km-wide escarpment zone east to east-west (Moseley, 1969; Fig. 14), This unconformity is an erosion surface bounding the Tihama Plain, suggesting that and this may be related to Gulf of Aden formed by basement uplift to the north, this was a preferred locus for magmatic in- rifting. which dies out eastward of A1 Tawilah jection (Fig. 14; Mohr, 1991). The majority (Fig. 4). The Cenozoic-age granite plutons of dikes throughout western Yemen are ori- EXHUMATION HISTORY terminate against this east-west contact ented broadly north-northwest-south- (Fig. 4). It is not clear whether a fundamen- southeast to northwest-southeast, indicating The amount of uplift during the Cenozoic tal structural discontinuity at deeper crustal east-northeast- to northeast-southwest-ori- can be determined from the present-day sur- levels has controlled this east-west flexural ented extension, which is slightly different face elevation of the marine beds in the up- trend, but there is no clear surface expres- but broadly compatible with the extensional per part of the Tawilah Group. The highest sion of any structural discontinuity. fault orientations (Fig. 14; Mohr, 1991). Lo- elevations that have been recorded for these cal dike trends are very variable in compar- sedimentary rocks in the traverse area is Dike Patterns ison to the main dike swarms mapped on 2560 m on Jabal Marmar. Apatite fission- satellite and aerial photographs (Fig. 14). track ages from basement amphibolite in Abundant mafic and felsic dikes cut all This variation is to be expected due to lo- Wadi La'ah, 5 km north of A1 Mahwit, are formations, from the Precambrian basement calized magmatic intrusions. Regional interpreted to indicate rapid exhumational

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Figure 12. (A) Geo- logic map of Bajil area, bordering the Tihama Plain (see Fig. 4 for location). Partly based on Kruck and others (1984). (B) West-southwest- east-northeast section through tilt blocks on the Tihama Plain, illustrat- ing east-dipping beds and west-dipping, domino-style faults (section is located in Fig. 12A).

wsw J. Falafilah J. al. Dhamir

-1000 s s *r \ \ j»\\\\\\\\\ \ sA \ \ ssy y y v y y y y y y s y y^s y * "

Miocene-Recent I I A m ran 2 km

I - -I Tertiary volcanic Khoian Horizontal = Vertical r.~.1 rocks

B mi Tawiiah Precambrian

cooling caused by erosional unroofing be- Jurassic strata (Akbra and Kohlan) between TIMING OF SEDIMENTATION, tween the onset of volcanism at 24-29 Ma Hajjah and Kohlan show evidence of total MAGMATISM, EXTENSION, and 16 ± 2 Ma (1 a) (Menzies and others, resetting. Resetting indicates that the strata AND UPLIFT 1992). Such exhumation postdated the ex- reached temperatures >110 °C prior to extensive flood volcanism and was probably humation. Minimum inferred burial and ex- Field and geochronologic data indicate produced by uplift of the Red Sea flanks. humation depths range between 1 and 4 km that magmatism preceded crustal extension Rifting would have increased the rugged- since 20 ± 5 Ma, as indicated by the central and uplift. K/Ar data indicate magmatism ness and elevation of the highest peaks, thus ages of a number of samples, and assuming began ca. 24-29 Ma in the transect area enhancing the potential for erosion. a geothermal gradient range of 30-100 (Civetta and others, 1977; Menzies and oth- Fission-track studies of the Permian to °C/km. ers, 1991). Large extensional faults are not

1488 Geological Society of America Bulletin, November 1994

LEGEND

Tertiary granite 12 W. Lahima sM^ . Figure 13. (A) Geo- Al MarwiK "--» > N Tertiary \ volcanic logic map of the area rocks around A1 Mahwit (see Tawilafi Fig. 4 for location). (B) mm Northeast-southwest Amran section south of AI Mah- wit, indicating west- + ::%-:-:/-Trv\ \ «VX _ Kholan dipping domino tilt v-x blocks intruded by gran- Precambrian ite to the southwest (see \...... « Fig. 13A for location). Lahima, Normal fault Location of the log Ft through Tawilah-Yemen P 27^ Bedding Volcanic Group contact .SWVV.V.V.V.'tt • in Figure 9 is shown as A * * A A A M A A A .A A A m A A A A AAA A A - « ¡Í* j* i Lilhologrc Tr. conlact

5 km Dyke

Wadial Wadi Wadi Wadi Aquf Rais Lahima Lahima Onset of Extension

fcs Ss \ s \ s . / S s

r . - . . / • • " • ' • "

Yemen volcanic rocks | | Amran Formation Granite

Ijljijlpjiil Tawilah Formation Kholan Formation I'V-/^ Precambrian Basement

recognized, which were active during depo- records indicate that medium-magnitude probably controlled by convection currents sition of the Amran and Tawilah strata and earthquake swarms are still common in induced by the continental separation that eruption of the Yemen Volcanic Group. Yemen (Ambraseys and Melville, 1983; was not symmetric with respect to the The main extension related to Red Sea rift- Plafker and others, 1987; Fig. 2). The last present-day Red Sea spreading center ing took place soon after deposition of the large earthquakes occurred in 1982 in the (Dixon and others, 1989). youngest fault-rotated Yemen Volcanic Dhamar area (Fig. 2; Ms = 5.7) when -1900 The Gulf of Aden has recently evolved Group (K/Ar ages of 18 Ma), because they people were killed, and in 1991 at Al Udain from continental rift to oceanic drift with are unconformably overlain by flat-lying late (Ms = 4.5), which caused severe damage to the formation of first oceanic crust ca. 10 Ma Miocene sedimentary rocks, and flat-lying property. at the eastern end of the Sheba Ridge and lO-m.y.-old (K/Ar ages) volcanic rocks 3-4 Ma west of long. 45°E (Cochran, 1981). (Huchon and others, 1992). The crust DISCUSSION This is similar in age to the estimated 5-6- stretched over an exposed onshore part —75 m.y.-old oceanic crust formed in the central km wide by a factor of —1.7, which gives The distribution of Cenozoic volcanic part of the Red Sea (between lat. 15° and an average extension rate of 4.5 mm/yr over rocks in the Arabian Peninsula cannot be 21°N; Roeser, 1975; Izzeldin, 1987; Fig. 15). an 8 m.y. period. Present-day extension is simply linked to a single vertical plume, be- The Gulf of Aden rifting appears to have now probably confined to the central axis of cause the onshore magmatic belt bordering propagated from the Carlsberg Ridge the Red Sea where thermal weakening has the Red Sea is a 300-km-wide zone that ex- toward the Afar hot spot (Fig. 15). The di- localized the deformation (Buck and others, tends for 3000 km northward from the Afar rection of propagation of the Red Sea Rift 1988). Some onshore deformation is con- triple junction to the Bitlis suture in Turkey is still not clearly defined, and several early tinuing at present, however, and historical (Fig. 15). The later volcanism (<10 Ma) was separated rift basins have been proposed

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Figure IS. Generalized geologic map of Middle East showing main Cenozoic volcanic areas, which define linear zones. Note northeast-trending rifts of Late Cretaceous to early Tertiary age in Ethiopia and Sudan. Elongate nature of the Afro-Arabian domal uplift is probably attributed to mantle convection cells associated with Red Sea rifting, as well as the Afar plume. Data from Al- mond (1986a, 1986b), Baldridge and others (1991), Bohannon and others (1989), Camp and Roobol (1989), Hart and others (1989), Mohr and Zanettin (1988), Omar and others (1987, 1989), and Voggenreiter and Hotzl (1989). •

icant weakening of the upper lithosphere probably occurred due to heating and magmatic intrusion, and regional stresses may have caused Red Sea rifting to propagate from the plume center and form an elongate rift, which propagated out from the weakened zone. Consistently oriented regional crustal strains are inferred from fault and dike orientations, and a hot spot alone may not have been enough to generate continental extension, as was the case in the huge Siberian flood basalt province, which has ap- parently no direct link with crustal extension (Coffin and Eldholm, 1992). The plume was able to produce significant surface volcanism (~2-3-km-thick volcanic rocks) before any evidence of major extensional faulting or uplift occurred. This may be explained if efficient magma transferral, from the melting plume head to the surface, occurred before the lithosphere was heated sufficiently to cause significant uplift (com- pare with Spohn and Schubert, 1983). This proposal is supported by the relatively small volume of dike injection (<1% of rock volume above sea level) observed in the prevolcanic section compared to the large thickness of the volcanic sequence. The timing of the onset of surface uplift and associated Figure 14. Dike orientations from our own data along the transect. Two rose diagrams denudation is not known, but apatite fission- south of the transect are from Moseley (1969), and major dike trends outside the transect track ages indicate that up to 3 km of den- from Mohr (1991). udation has occurred since 16 Ma. In sum- mary, our observations differ from the farther north in Saudi Arabia (Brown and The field evidence strongly suggests that classic modeled chronology of active rifting others, 1989) and the Gulf of Suez (Makris rifting took place due to the impingement of where surface uplift precedes volcanism and and Rihm, 1991). The timing of the onset of a plume, because massive volcanism took extension (Sengor and Burke, 1978; White extension and magmatism appears to be syn- place before any upper crustal extension. and McKenzie, 1989). The early long sag chronous along most of the Red Sea margin The geochemical evidence (Baker and oth- phase marked by the deposition of the Am- (Menzies and others, 1992; and references ers, 1993a, 1993b) and seismic tomography ran and Tawilah Groups may represent a therein), suggesting that Red Sea Rift prop- interpretations (Zhang and Tanimoto, phase of ductile extension in the lower litho- agation was relatively rapid. 1992) also point to a plume source. Signif- sphere, with extensional elastic strain in the

1490 Geological Society of America Bulletin, November 1994

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EASTERN LIMIT OF ROTATED FAULT Sana a EASTERN LIMIT OF BLOCKS , VOLCANIC ROCKS RED SEA Granite YEMEN PLATEAU Coastline batholith Elevation 2.5 km km r 0 T 10 :20

UJ - 30 Dike Intrusion ? -Ujj_ i J_L J .1. L J THINNED CRUST P 40 MOHO TRANSITION ZONE ß= 1.6- 1.8 50 km Horizontal = Vertical

Figure 16. East-west schematic crustal scale section from east of Sana'a to center of the Red Sea. Crustal thickness from Egloff and others (1991) and Makris and others (1991). Offshore structure interpreted from British Petroleum proprietary seismic data, and onshore structure from our own observations.

upper crust, where the elastic limit was not Sea volcanism ca. 30 Ma. The sedimentation ology at Royal Holloway is acknowledged. reached (Lynch and Morgan, 1990). rate broadly kept pace with subsidence rate BP exploration and the Yemen Republic Magmatism appears to be spatially re- such that the Permian to early Tertiary Petroleum Exploration Board provided per- lated to higher surface elevation, with most strata are mainly shallow marine to conti- mission to use seismic lines across the of the area lying above 2000 m being vol- nental. Angular unconformities, or evidence Tihama Plain. Sana'a University is thanked canic or plutonic (Fig. 2). We attribute the of deformation, were not observed through- for its logistical help and transportation. present-day elevation of the Yemen high- out the prevolcanic Mesozoic sedimentary Baker was supported by a British Common- lands to a combination of magmatic under- sequence. wealth Scholarship. Blakey holds a Natural plating, extrusion of 2-4 km thickness of vol- (2) Volcanism predated important crustal Environment Research Council postgradu- canic rocks at the surface, the buoyancy extension, reaching a climax between ca. 30 ate studentship. McClay and Dart were sup- uplift of the plume, and the high heat flow and 25 Ma. ported by the Fault Dynamics Project at producing thermal expansion of the litho- (3) The regional extension direction was Royal Holloway. Fieldwork by Davison and sphere. High heat flow in the Al-Auch-1 well mainly northeast-southwest, and remained Owen on the Great Escarpment was funded 2 of 110 mW/m has been recorded on the broadly consistent from the onset of rift by a European Community Red Sea grant to Tihama Plain (Barnard and others, 1992). magmatism to the present day. B. Purser, Université de Paris, with Bosence Gravity and seismic refraction data indicate (4) Red Sea rifting produced an 80-120- as United Kingdom Coordinator. Fission- that crustal thickness is —35 km below the km-wide extended zone across its eastern track research is supported by NERC re- central Yemen Mountains, which excludes margin with an extension factor (3 varying search grant GR3/8457 (Menzies and Hur- any very large magmatic underplating of between 1.6 and 1.8. Extension took place in ford), and analyses were carried out at the crustal density (Fig. 16, Egloff and others, the middle to late Miocene (ca. 18-10 Ma). London Fission Track Research Group 1991; Makris and others, 1991). It is also (5) Surface uplift is attributed to plume laboratories, University College, London. worth noting that thick (2 km) volcanic heating, and apatite fission-track ages imply K. Al-Thour gave permission to use infor- rocks crop out up to 180 km east of the ex- a minimum age for exhumational cooling ca. mation from his Ph.D. thesis. T. Dooley and tended terrane (Figs. 1 and 16), suggesting 16-20 Ma contemporaneous with rifting. L. Blything are thanked for their help with drafting the diagrams. We also thank that there is no direct spatial link between (6) The classic active and passive rifting P. Morgan, A. Aydin, A. Sylvester, and an crustal extension and volcanism in this part models clearly do not fit the geologic evi- anonymous reviewer for their very helpful of the Red Sea margin. Crustal thinning cal- dence, which is magnificently exposed along comments. culated from geophysical data reaches a the Yemen-Red Sea margin. We suggest maximum stretching factor ¡3 of 2.4 beneath that an active plume weakened the litho- the Red Sea (Makris and others, 1991; sphere and allowed rifting to proceed due to REFERENCES CITED Fig. 16). The position of upper crustal ex- regional plate stresses, which would have tension, with major surface faulting, appears Al-Subbary, A., 1990, Stratigraphie and sedimentological studies been otherwise too weak to produce conti- of the Tawilah Formation, Al-Ghiras area, northeast to coincide vertically with the total crustal Sana'a, Yemen Arab Republic [Master's thesis]: Sana'a, nental separation (Lynch and Morgan, Yemen Arab Republic, University of Sana'a, 184 p. thinning indicated by the gravity and seismic Al-Subbary, A., and Nichols, G. J., 1991, Cretaceous-early Ter- 1990). tiary pre-rift sediments, Yemen: British Sedimentological refraction data, indicating a pure shear pat- Research Group Annual Meeting, Edinburgh, Abstracts, no tern of crustal thinning in this region page number. ACKNOWLEDGMENTS Al-Subbary, A., Nichols, G. J., and Bosence, D., 1994, Cretaceous (Fig. 16). to Tertiary pre-rift fluvial to shallow marine sediments in Yemen: Egyptian Journal of Geology (in press). We thank the Royal Society for an Over- Al-Thour, K. A. A., 1992, Stratigraphy, sedimentology, and di- agenesis of the Amran Group (Jurassic) of the region to the CONCLUSIONS seas Travel Grant (Bosence and others). west and north-west of Sana'a, Yemen Republic [Ph.D. thesis]: Birmingham, United Kingdom, University of Birming- Additional support from the British Coun- ham, 293 p. (1) Subsidence of ~1 km occurred from cil, British Petroleum (BP), and the Indus- Almond, D. C., 1986a, Geological evolution of the Afro-Arabian Dome: Tectonophysics, v. 131, p. 301-332. Permian times through to the onset of Red trial Association of the Department of Ge- Almond, D. C., 1986b, The relation of Mesozoic-Cainozoie vol-

1492 Geological Society of America Bulletin, November 1994

canism to tectonics in the Afro-Arabian Dome: Journal of Davison, I., Alsop, I., Bosence, D., and Al-Aawah, M. H., 1994, Gasparon, M., La Volpe, L., and Orsi, G., 1991, Magmatism Volcanology and Geothermal Research, v. 28, p. 225-246. Deformation and sedimentation around exhumed Miocene of the eastern Red Sea margin in the northern part of Ambraseys, N., and Melville, C. P., 1983, Seismicity of Yemen: salt diapirs, A1 Salif and Jabal al Milh, Tihama Plain, NW. Yemen from Oligocene to present: Tectonophysics, v. 198, Nature, v. 303, p. 321-323. Yemen: Salt Tectonics Conference, 14-15 September, Ge- p. 181-202. Baker, J. A., Thirlwall, M. F., and Menzies, M. A., 1993a, Sr, Nd ological Society of London, Abstracts, no page number. McGuire, A. V., and Bohannon, R. G., 1989, Timing of mantle and Pb isotope geochemistry of Tertiary flood volcanism in Dixon, T. H., Ivins, E. R., and Franklin, B. 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